This invention relates to superconductive material and apparatus and systems utilizing such material. Characteristics of preferred compositions herein give rise to apparatus design advantages. Such characteristics include useful values of critical temperature and critical magnetic field. Certain of the compositions are free of characteristics which have been associated with radiation damage in prior materials.
The field of superconductivity has had a varied history from its beginningsxe2x80x94from the discovery of the phenomenon by Kamerlingh Onnes in 1911. Implications of extreme practical significance were apparent from the start. Workers became enchanted with the thought of lossless transmission as well as implications on magnetic and other apparatus. The concept that has received so much attention in recent yearsxe2x80x94that of xe2x80x9cpermanentxe2x80x9d magnets of extremely high field values useful e.g. for containment of fusion reactionsxe2x80x94was not overlooked.
Advances in obtaining material with improved properties have been discontinuous. Experiments following the first discovery, while certainly substantiating the concept, established need for extremely low temperatures, i.e., low values of transition temperatures, Tc. Mercury (Tcxcx9c4 K) was discovered first, and lead (Tcxcx9c7 K) was discovered to be superconducting shortly thereafter.
There was little achievement either experimentally or in theoretical terms following the initial period until the 1940""s when work on NbN yielded a Tc value of about 16K. Evolution and description of the concept was not broadly and effectively disseminated because of its German origin and circumstances surrounding World War II. A significant period of inquiry began subsequent to World War II. From the mechanistic standpoint, the most significant advance entailed identification of Type II superconductivity (Ginzburg and Landau, 20 Zh. Eksperim. i Teor. Fiz., pg. 1064 (1950)).
From a mechanistic-theoretical standpoint 1957 marks a most significant advance in understanding of superconductivity. The Bardeen Cooper Schrieffer Theory (for which a Nobel prize was subsequently awarded), (108 Phys. Rev., pg. 1175 (1957)) gave rise to the understanding which has been implicit in all studies to the present time. Intensive worldwide effort was established by the activity of B. T. Matthias of Bell Laboratories and J. K. Hulm of Westinghouse resulting in the most significant A15 compounds exemplified by Nb3Sn. This was followed by identification of the members as well as of related alloy compositions generally containing Nb. This work gave renewed hope of practical applications working with a refrigerant more effective than liquid helium. Materials developed during this period continue to be studied extensively, and indeed serve very significant technological functions, e.g., in particle accelerators.
Efforts to develop materials with significantly higher values of Tc than that of Nb3Sn(Tc=18 K) were disappointing. Probably the culmination was the Tc value of xcx9c23.2K for Nb3Ge in 1973 by L. Testardi at Bell Laboratories and J. R. Gavaler at Westinghouse.
The subsequent decade resulted in little advance in terms of Tc. Extensive study of a new category of material was, however, very important and plays a vital role in development of this invention. This study entailed compositions of barium bismuth lead oxide (BaPb1xe2x88x92BizO3). B. Batlogg, Physica 126B, 275 (1984). Most significant, compositions in this category while attaining Tc values of only up to 13K depended upon metal-oxygen bonding for superconductivity.
Most recent origins of the present invention are traceable to the extremely significant work reported by IBM Zurichxe2x80x94J. G. Bednorz and K. A. Muller (64 Z. Phys. B.xe2x80x94Condensed Matter, pp. 189 (1986)). Report of onset Tc values in the 30K range in the Laxe2x80x94Baxe2x80x94Cuxe2x80x94O system stimulated intense activity by a number of groups worldwide.
Significant consequences of work stimulated by the above are reflected in recent articles.
Workers at the University of Tokyo in Japan J. Appl. Phys., significantly advanced understanding of the IBM work by identification of the superconducting phase as being of the K2NiF4 structure. The composition on which measurements were made in accordance with this publication was that of the IBM Zurich report.
C. W. Chu and co-workers at the University of Houston reported on the beneficial effect of application of hydrostatic pressure during measurement in Phys. Rev. Lett., 58, 405 (1987). In a revision to this article made subsequent to the original submission date, comment was made on the substitution of strontium for barium.
The same issue of Phys. Rev. Lett., 58 at page 408, reported true bulk conductivity at 39.2 K in the composition La1.85Sr0.15CuO4. This finding constitutes a portion of the disclosure in parent U. S. patent application Ser. No. 001,682, filed Jan. 9, 1987.
The published works were accompanied and followed by a series of tantalizing rumors reporting xe2x80x9consetxe2x80x9d temperatures well above the 77 K boiling point of liquid nitrogen. The March 2 issue of Phys. Rev. Lett., (Vol. 58, 1987), contains two extremely significant papers representing advances prompted by the new findings. At page 908 et seq., workers at the Universities of Alabama and Houston report on compositions in the Yxe2x80x94Baxe2x80x94Cuxe2x80x94O system evidencing Tc values well above 77 K. Their exemplary composition (Y0.6Ba0.4)2CuO less than 4 is described as multiple phase (xe2x80x9cinstead of the pure K2NiF4 phasexe2x80x9d [associated with previously reported compositions]). Reporting a deviation in resistivity from the usual temperature relationship which is initiated at 93 K, the paper goes on to report attainment of zero resistivity at 80 K. Rather than associated attainment of this value with the particular crystallographic phase, authors state xe2x80x9c . . . high temperature superconductivity may be associated with interfacial manifestationsxe2x80x9d. At page 911 in the same issue of Phys. Rev. Lett., the same authors in a separate article reached the specific conclusion xe2x80x9cIt is therefore evident that . . . above 77 K [Tc] may not be identified with . . . perovskite or . . . tetragonal layered structures . . . xe2x80x9d. Developments to date have taken on the drama of an international race with runners representing essentially every organization in any way affiliated with superconductivity. Events to date are reflected in an article on page 12 of the Star-Ledger for Tuesday, Mar. 3, 1987. In this article, world-famous physicist John Rowell, Assistant Vice President of Bell Communications Research, reports as their xe2x80x9cinternational contributionxe2x80x9d the xe2x80x9c . . . producing results identical to those achieved in general by scientists at the University of Houston . . . xe2x80x9d. Dr. Roweil explicitly confirms the multiphase nature of the superconducting material and, in fact, reports that xe2x80x9cOnly about 2% . . . is superconducting . . . xe2x80x9d.
An important aspect of the invention takes the form of identification and isolation of the single crystallographic phase responsible for the superconducting characteristics that have been the subject of the world-wide quest. In a general sense, identification is of a perovskite or near-perovskite (referred to collectively as xe2x80x9cperovskitexe2x80x9d) phase of specific composition. All included materials of the xe2x80x9cperovskitexe2x80x9d class are primarily pure quaternary or partially substituted quaternary copper oxides. Nominal compositionsxe2x80x94all single phase xe2x80x9cperovskitexe2x80x9dxe2x80x94may be represented by the general formula M3xe2x88x92mMxe2x80x2mCu3O9xe2x88x92xcex4, although specific materials of the general compositions of the reported multiphase material (Phys. Rev. Lett., Vol. 58, page 908 et seq., Mar. 2, 1987), are of the more specific nominal composition M2Mxe2x80x21Cu3O9xe2x88x92xcex4. The latter materials are based on a nominal 2:1 atomic ratio of divalent:trivalent cation inclusion in the M site. From the compositional standpoint, consistent with prior reported work, a class of materials of the invention are those in which M is predominantly barium and M is predominantly yttrium. A more general class of materials of the invention are those of the first formula above in which M occupancy while averaging divalency, may contain an additional divalent ion such as strontium or calcium or in which the average valence state may be the consequence of occupancy by ions of differing valence values. Specific compositions described herein lack use of 1:1 occupancy by trivalent and monovalent (rare earth and alkali metal) ions. A class of materials of the invention depend upon predominant occupancy of Mxe2x80x2 by yttrium, although improved Tc values have been obtained by total substitution of europium. Other permitted Mxe2x80x2 ions are lutetium, lanthanum and scandium. With the exception of strontium and scandium and, of course, where valence averaging is intended, superconducting compositions of the invention have been based on total as well as partial M and/or Mxe2x80x2 occupancy.
Most useful superconducting properties have, in many instances, been traced to average copper valence (Cun+=2.0 less than n less than 2.67). Oxygen content has been found to lie between the limits of 6.5 and 7.5 (1.5 less than xcex4 less than 2.5) for analyzed compositions herein.
Inventive implications apply to essentially all superconducting apparatus. The inventive findings in terms of preferred compositions have already provoked restudy of long-distance power transmission now made possible at liquid nitrogen temperatures. Other implications, of course, arise from the higher values of critical field with implications to fusion reaction containment, for example. Such purposes are consistent with materials characterized by continuous superconducting paths. Other purposes also well-known to prior workers include use in devices such as detectors and those dependent on tunneling phenomena such as Josephson junction devices as well as other devices which depend on weakly coupled superconductors and/or in the large value of the superconducting gap. Application of particular value has been recognized in the literature (IBM Journal of Research and Development, Vol. 24, No. 2, March 1980). The desire to decrease computing time which has provoked study of optical computers may take an alternate form in which interconnections, e.g., in a semiconductor computer, may be superconducting. Absence of resistance in a superconductor permits scaling down to result in a feasibly sized transmission line interconnect. In fact, use of compositions herein with Tc values sufficiently high to permit liquid nitrogen refrigerant is compatible with already-suggested increasing semiconductor mobility by liquid nitrogen cooling.